Alignment of semiconductor wafers is usually achieved by utilizing mechanical or optical fiducial marks, such as notches at the edges of wafers, pins, e-beam etchings, or holographic images, among others. Mechanical alignment marks provide millimeter range alignment accuracy, while optical marks provide micron to submicron alignment accuracy.
In several semiconductor processes, such as 3-D integration, bonding and mask alignment, among others, submicron to nanometer alignment accuracy is desirable.